Gaëlle Delaizir

Effective medium theory based modeling of the thermoelectric properties of composites: comparison between predictions and experiments in the glass-crystal composite system Si10As15Te75–Bi0.4Sb1.6Te3

We report on the theoretical predictions of the effective medium theory (EMT) and its generalized version taking into account percolation theory (GEMT) on the thermoelectric properties of composites based on Landauer and Sonntags equations. The results were tested experimentally on composites composed of the glassy phase Si10As15Te75 and the crystalline phase Bi0.4Sb1.7Te3. The evolution of the electrical resistivity and thermal conductivity with the fraction of crystalline phase matches very well the experimental data, although the GEMT model fails to predict the thermopower. A better agreement between theory and experiment could be obtained by combining the principles of the GEMT and the Webman–Jortner–Cohen models. Despite the fact that the GEMT model originally predicts the possibility to optimize the dimensionless figure of merit ZT of composites by adjusting the fraction and the values of the transport properties of each phase, the new model developed rules out any beneficial influence on the ZT values. These results confirm within a different framework the early conclusions of Bergman regarding the impossibility of improving the ZT values using multi-phased materials.

High thermoelectric performance in Sn-substituted α-As2Te3

Lead chalcogenides PbX (X = Te, Se, S) have been the materials of choice for thermoelectric power generation at mid-range temperatures (500–700 K). Here, we report on a new family of chalcogenides α-As2Te3 that exhibit similar thermoelectric performances near 500 K. Sn doping in p-type polycrystalline α-As2−xSnxTe3 (x ≤ 0.075) provides an efficient control parameter to tune the carrier concentration leading to thermopower values that exceed 300 μV K−1 at 500 K. Combined with the structural complexity of the monoclinic lattice that results in extremely low thermal conductivity (0.55 W m−1 K−1 at 523 K), a peak ZT value of 0.8 is achieved at 523 K for x = 0.05. A single-parabolic band model is found to capture well the variations in the transport properties with the Sn concentration and suggests that higher ZT values could be achieved through band structure engineering. These results surpass those obtained in the sister compounds β-As2−xSnxTe3 and further show that α-As2Te3 based materials are competitive with other chalcogenides for thermoelectric applications at intermediate temperatures.

Thermoelectric Properties of the alpha-As2Te3 Crystalline Phase

As2Te3 exists in two crystallographic configurations: α- and β-As2Te3, of which only the latter crystallizes in the same rhombohedral structure-type as Bi2Te3. While β-As2Te3 shows interesting thermoelectric (TE) properties which can be adjusted through alloying, the transport properties of the monoclinic phase α-As2Te3, more thermodynamically stable than the β-phase at room temperature, has not yet been studied thoroughly. We report here on the samples preparation by powder metallurgy, and on the microstructural characterization of polycrystalline α-As2Te3. Preliminary results on the electrical and thermal properties measured between 5xa0K and 523xa0K are also reported. Transport properties measurements were performed both along and perpendicular to the pressing direction indicating that the transport properties demonstrate some degree of anisotropy. Remarkably, low thermal conductivity values (below 1xa0Wxa0m−1xa0K−1 above 300xa0K) were measured suggesting that this compound may be an interesting platform to design novel TE materials with high efficiency.

Electronic structure, low-temperature transport and thermodynamic properties of polymorphic β-As2Te3

β-As2Te3 belongs to the prominent family of Bi2Te3-based materials, which show excellent thermoelectric properties near room temperature. In this study, we report a joint theoretical and experimental investigation of its electronic and thermal properties at low temperatures (5–300 K). These results are complemented by specific heat measurements (1.8–300 K) that provide further experimental evidence of the first order lattice distortion undergone by β-As2Te3 near 190 K. Data taken on cooling and heating across this transition show that the lattice distortion has little influence on the electronic properties and further evidence a weak hysteretic behavior. Although first-principles calculations predict a semiconducting ground state, these measurements show that β-As2Te3 behaves as a degenerate p-type semiconductor with a high carrier concentration of 1020 cm−3 at 300 K likely due to intrinsic defects. Calculations of the vibrational properties indicate that the extremely low lattice thermal conductivity values (0.8 W m−1 K−1 at 300 K) mainly originate from low-energy Te optical modes that limit the energy window of the acoustic branches. This limited ability to transport heat combined with a relatively large band gap suggest that high thermoelectric efficiency could be achieved in this compound when appropriately doped.

Low-Temperature Transport Properties of Bi-Substitutedβ-As2Te3 Compounds

Abstractβ-As2Te3 belongs to the family of Bi2Te3-based alloys, a well-known class of efficient thermoelectric materials around room temperature. As2Te3 exists in two allotropic configurations: α- and β-As2Te3, of which only the latter crystallizes in the same rhombohedral structure as Bi2Te3. Herein, we report on substitution of Bi for As in the As2−xBixTe3 system with xxa0=xa00.0, 0.015, 0.025, and 0.035. These samples have been characterized by x-ray diffraction and scanning electron microscopy. The transport properties have been measured at low temperatures (5xa0K to 300xa0K) in both directions, parallel and perpendicular to the pressing direction. The results are compared with those obtained in previous study on samples substituted by Sn. Compared with Sn, Bi allows for a clear decrease in electrical resistivity while maintaining the thermal conductivity below 1xa0W/(mxa0K) over the whole temperature range. As a result, a comparable peak ZT value near 0.2 was obtained at room temperature.

High-temperature thermoelectric properties of the β-As2−xBixTe3 solid solution

Bi2Te3-based compounds are a well-known class of outstanding thermoelectric materials. β-As2Te3, another member of this family, exhibits promising thermoelectric properties around 400 K when appropriately doped. Herein, we investigate the high-temperature thermoelectric properties of the β-As2−xBixTe3 solid solution. Powder X-ray diffraction and scanning electron microscopy experiments showed that a solid solution only exists up to x = 0.035. We found that substituting Bi for As has a beneficial influence on the thermopower, which, combined with extremely low thermal conductivity values, results in a maximum ZT value of 0.7 at 423 K for x = 0.017 perpendicular to the pressing direction.

Fabrication of optical fibers with palladium metallic particles embedded into the silica cladding

Absorption of hydrogen gas (H2) in contact with palladium (Pd) makes Pd a material of choice for numerous H2 sensors. In this paper, we present the fabrication of optical fibers with embedded Pd particles in the silica cladding of the fibers. Fiber preforms prepared with a powder mixture of silica and palladium oxide (PdO) are heat-treated under specific conditions to reduce PdO to metallic Pd particles, dispersed in the silica matrix. Optical fibers with different topologies have been fabricated with lengths of several hundred meters and PdO concentration ranging from 0.01% to 5% mol (in addition to silica). Oxidation state, homogeneity, shape and size distribution of the particles embedded in the cladding of the preform and the fiber samples are studied with structural and micro-structural characterizations. Optical properties of the fibers are finally studied for evaluating the potential of this proof-of-concept work.

Multimaterial polarization maintaining optical fibers fabricated with the powder-in-tube technology

We present the fabrication of multimaterial polarization maintaining optical fibers. We exploit the flexibility of the powder-in-tube process for fabricating silica-based optical fibers composed of two rods of glass material on the sides of the core. We demonstrate the capability of this process to use glass material with properties sorely different from the ones of silica for developing high-birefringence optical fiber. This proof-of-concept paves the way for the use of different materials with specific properties for improving the performances of polarization maintaining optical fibers.

Semiconducting glasses: A new class of thermoelectric materials?

This paper reviews the semiconducting glasses for thermoelectric applications. Several examples of tellurium-based glasses, with high Seebeck coefficients, very low thermal conductivities and tunable electrical conductivities, are presented, pointing this family of glasses as a good candidate for high-performance thermoelectric materials.